Method for manufacturing compound semiconductor device

ABSTRACT

A semiconductor layer ( 2,3 ) is provided on a substrate ( 1 ). A gate electrode ( 4 ), a source electrode ( 5 ) and a drain electrode ( 6 ) are provided on the semiconductor layer ( 3 ). A first passivation film ( 7 ) covers the gate electrode ( 4 ) and the semiconductor layer ( 3 ). A source field plate ( 9 ) is provided on the first passivation film ( 7 ), and extends from the source electrode ( 5 ) to a space between the gate electrode ( 4 ) and the drain electrode ( 6 ). A second passivation film ( 10 ) covers the first passivation film ( 7 ) and the source field plate ( 9 ). An end portion on the drain electrode ( 6 ) side of the source field plate ( 9 ) is curved to be rounded.

FIELD

The present invention relates to a compound semiconductor device that ishard to be broken and deteriorated even under a severe environment underwhich the compound semiconductor device is exposed to high energyparticles, and a manufacturing method capable of easily manufacturingsuch a device.

BACKGROUND

A compound semiconductor device is used as a field effect transistorsuch as a MES-FET or a HEMT (for example, see PTL 1 to PTL 4). There isa ease where a device is exposed to a severe environment under whichhigh energy particles are incident to the device, pass through apassivation film, a source field plate, and an active region of thedevice and reach a substrate. At this time, a large amount ofelectron-hole pairs are generated around a trajectory through which thehigh-energy particles have passed, and diffused or recombined accordingto the mobility of material, a recombination speed, and an appliedvoltage.

CITATION LIST Patent Literature

-   [PTL 1] JP 2006-253654 A-   [PTL 2] JP 2008-243943 A-   [PTL 3] JP 2010-67693 A-   [PTL 4] JP 2015-170821 A

SUMMARY Technical Problem

A high electric field is applied between an end portion on a drainelectrode side of a source field plate and an AlGaN channel layer.Therefore, when a large amount of electron-hole pairs are generated inthe passivation film upon incidence of high-energy particles, aconduction path is formed at that portion, resulting in breakage. Or,there has been a problem that the concentration of holes in the vicinityof the surface of a semiconductor increases in the process of diffusionand recombination of electron-hole pairs generated in the semiconductor,which causes an increase in potential or an increase in hole current,resulting the semiconductor device becoming prone to breakage ordeterioration. Likewise, there has been a problem that a high electricfield is applied between an end portion on a drain electrode side of agate electrode and the AlGaN channel layer, which makes thesemiconductor device prone to breakage or deterioration.

Therefore, it has been performed that the end portion of the sourcefield plate is bent upward at a certain angle to relax the electricfield, thereby preventing breakage of the device (for example, see FIG.6 and paragraph 0043 of PTL 1, and FIG. 1B and paragraph 0015 of PTL 3).However, an electric field relaxation effect is also limited because abent portion exists. Furthermore, the conventional method have had aproblem that a complicated step is required to be added to bend thesource field plate, manufacturing is difficult, and the number ofmanufacturing steps increases, thereby increasing the cost andmanufacturing time.

The present invention has been made to solve the problems as describedabove, and has an object to obtain a compound semiconductor device thatis hard to he broken and deteriorated even under a severe environmentunder which the compound semiconductor device is exposed to high energyparticles, and a manufacturing method capable of easily manufacturingsuch a device.

Solution to Problem

A compound semiconductor device according to the present inventionincludes: a substrate; a semiconductor layer provided on the substrate;a gate electrode, a source electrode and a drain electrode provided onthe semiconductor layer; a first passivation film covering the gateelectrode and the semiconductor layer; a source field plate provided onthe first passivation film, and extending from the source electrode to aspace between the gate electrode and the drain electrode; and a secondpassivation film covering the first passivation film and the sourcefield plate, wherein an end portion on the drain electrode side of thesource field plate is curved to be rounded.

Advantageous Effects of Invention

In the present invention, the end portion on the drain electrode side ofthe source field plate is curved to be rounded. Accordingly, there is noprotruding portion and also it is possible to sufficiently perform theelectric field relaxation, so that the device is hard to be broken anddeteriorated even under a severe environment exposed to high energyparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a compound semiconductor deviceaccording to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a compound semiconductor deviceaccording to a comparative example.

FIG. 3 is a cross-sectional view showing a manufacturing process of thecompound semiconductor device according to the embodiment of the presentinvention.

FIG. 4 is a cross-sectional view showing a manufacturing process of thecompound semiconductor device according to the embodiment of the presentinvention.

FIG. 5 is a cross-sectional view showing a manufacturing process of thecompound semiconductor device according to the embodiment of the presentinvention.

FIG. 6 is a cross-sectional view showing a manufacturing process of thecompound semiconductor device according to the embodiment of the presentinvention.

FIG. 7 is a cross-sectional view showing a manufacturing process of acompound semiconductor device according to a comparative example.

FIG. 8 is a cross-sectional view showing a manufacturing process of acompound semiconductor device according to a comparative example.

FIG. 9 is a cross-sectional view showing a manufacturing process of acompound semiconductor device according to a comparative example.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a cross-sectional view showing a compound semiconductor deviceaccording to an embodiment of the present invention. A GaN buffer layer2 is formed on an SiC substrate 1. An AlGaN channel layer 3 is formed onthe GaN buffer layer 2. A gate electrode 4, a source electrode 5, and adrain electrode 6 are formed on the AlGaN channel layer 3.

A first passivation film 7 covers the gate electrode 4 and the AlGaNchannel layer 3. A source field plate 9 is formed on the firstpassivation film 7, and extends from the source electrode 5 to a spacebetween the gate electrode 4 and the drain electrode 6. The source fieldplate 9 relaxes the electric field between the gate electrode 4 and thedrain electrode 6, enables a high voltage operation, and further reducesa parasitic capacitance, thereby improving high frequencycharacteristics. In order to protect the entire device, a secondpassivation film 10 covers the first passivation film 7 and the sourcefield plate 9.

When a voltage is applied between the source electrode 5 and the drainelectrode 6, and a high frequency is input to the gate electrode 4 whilea desired bias voltage is applied to the gate electrode 4, electrons intwo-dimensional electron gas 11 move at a high speed, whereby thecompound semiconductor device operates as an amplifier capable ofobtaining amplified high frequency power from the drain electrode 6.

Subsequently, the effect of the compound semiconductor device accordingto the present embodiment will be described in comparison with acomparative example. FIG. 2 is a cross-sectional view showing a compoundsemiconductor device according to a comparative example. There is a casewhere upon incidence of high-energy particles to the device, thehigh-energy particles pass through the second passivation film 10, thesource field plate 9, the first passivation film 7, the AlGaN channellayer 3, and the GaN buffer layer 2 and reaches the SiC substrate 1.Incoming particles are heavy particles, protons, electrons, neutrons,muons, etc. and have energy of about 1 keV to 100 GeV. A large amount ofelectron-hole pairs are generated around a trajectory through which thehigh-energy particles have passed.

Normally, the electric field concentrates on a portion having a sharpangle. In the prior art, the end portion of the source field plate has aright angle, so that the electric field concentrates on the end portion.In the comparative example, in order to suppress this concentration ofthe electric field, the end portion of the source field plate 9 is bentupward at a certain angle. However, the electric field relaxation effectis also limited because a bent portion exists.

On the other hand, in the present embodiment, the end portion on thedrain electrode side 6 of the source field plate 9 is curved to berounded in an inversely tapered shape. Accordingly, there is noprotruding portion and also it is possible to sufficiently perform theelectric field relaxation, so that the device is hard to be broken anddeteriorated even under a severe environment exposed to high energyparticles. Although an upper side of the end portion of the source fieldplate 9 may be curved, the effect is limited because it is far from thesemiconductor. Accordingly, it is preferable that the end portion of thesource field plate 9 is inversely tapered.

FIGS. 3 to 6 are cross-sectional views showing a manufacturing processof the compound semiconductor device according to the embodiment of thepresent invention. First, as shown in FIG. 3, the GaN buffer layer 2 andthe AlGaN channel layer 3 are successively formed on the SiC substrate1. The gate electrode 4, the source electrode 5 and the drain electrode6 are formed on the AlGaN channel layer 3. The first passivation film 7covering the gate electrode 4 and the AlGaN channel layer 3 is formed.

Next, as shown in FIG. 4, a resist 12 extending from the drain electrode6 to a space between the gate electrode 4 and the drain electrode 6 isformed on the first passivation film 7. Next, as shown in FIG. 5, thesource field plate 9 is formed on the first passivation film 7 and theresist 12.

Next, as shown in FIG. 6, a lift-off step for removing the resist 12 andthe source field plate 9 on the resist 12 is performed. Since the resist12 is thickly formed, stepping occurs, and an unnecessary portion isalso removed by removing the resist 12. Thereafter, the secondpassivation film 10 covering the first passivation film 7 and the sourcefield plate 9 is formed.

Here, the resist 12 is, for example, a model number BL-300 of PIMEL(registered trademark) manufactured by Asahi Kasei E-Materials Co., Ltd.When the resist 12 is formed, a heat treatment is performed at 350° C.for 2 hours to shrink the resist 12, so that the side surface of theresist 12 is thermally drooped in a concave shape. When the source fieldplate 9 is formed in this state, the end portion on the drain electrode6 side of the source field plate 9 is curved to be rounded in aninversely tapered shape.

Subsequently, the effect of the manufacturing method according to thepresent embodiment will be described in comparison with a comparativeexample. FIGS. 7 to 9 are cross-sectional views showing a manufacturingprocess of a compound semiconductor device according to a comparativeexample. After performing the step of FIG. 3, a spacer film 13 having aslope shape is formed on the first passivation film 7 as shown in FIG.7. This slope shape is formed by isotropically etching or the like withdry etching after the resist is formed. Next, as shown in FIG. 8, thesource field plate 9 is formed on the first passivation film 7 and thespacer film 13. Next, as shown in FIG. 9, the source field plate 9 ispartially covered with the resist 14, and an unnecessary portion of thesource field plate 9 is removed by etching using the resist 14 as amask. Thereafter, the spacer film 13 and the resist 14 are removed, andthe second passivation film 10 is formed as shown in FIG. 2.

In the present embodiment, the end portion of the source field plate 9can be directly curved, so that the number of steps can be reduced byone as compared with the comparative example. Therefore, themanufacturing cost and the manufacturing period can be reduced.Furthermore, the curving work can be performed easily.

REFERENCE SIGNS LIST

-   1 SiC substrate; 2 GaN buffer layer; 3 AlGaN channel layer; 4 gate    electrode; 5 source electrode; 6 drain electrode; 7 first    passivation film; 9 source field plate; 10 second passivation film;    12 resist

The invention claimed is:
 1. A method for manufacturing a compoundsemiconductor device comprising: forming a semiconductor layer on asubstrate; forming a gate electrode, a source electrode and a drainelectrode on the semiconductor layer; forming a first passivation filmcovering the gate electrode and the semiconductor layer; forming aresist extending from the drain electrode to a space between the gateelectrode and the drain electrode on the first passivation film; forminga conductive film on the first passivation film and the resist; removingthe resist and the conductive film on the resist to form a source fieldplate; and forming a second passivation film covering the firstpassivation film and the source field plate, wherein when the resist isformed, a heat treatment is performed to shrink the resist so that aside surface of the resist is thermally drooped in a concave shape.